Method and apparatus for growing semi-conductive single crystals from a melt



June 2, 1959 os 2,889,240

METHOD AND APPARATUS FOR GROWING SEMI-CONDUCTIVE SINGLE CRYSTALS FROM AMELT Filed March 1, 1956 2 Sheets-Sheet 1 IN VEN TOR.

FRED D. Ros:

Byffgg June 2, 1959 F. D. ROS! METHOD AND APPARATUS FOR GROWING SEMI- 2Sheets-Sheet 2 CONDUCTI SINGLE CRYSTALS FROM A MELT Filed March 1, 1956INVENTOR. FRED D. Hus;

{rmmi/ nitecl tates Fred D. Rosi, Plainsboro, N.J., assignor to RadioCorporation of America, a corporation of Delaware Application March 1,1956, Serial No. 568,798

11 Claims. (Cl. 1481.6)

This application is a continuation-in-part of application Serial No.535,391, filed September 20, 1955, now abandoned.

This invention relates to improved methods and apparatus for growingsingle crystals of semi-conductive materials. More particularly, theinvention relates to improved crystal growing methods and apparatus forproducing single crystals, such as germanium or silicon, having fewerstructural imperfections and having high lifetime of minority chargecarriers, making them more suitable for semi-conductor devices.

The growing of single crystals of a material by solidifying successiveportions of the material from its molten phase is well known. Two basictechniques are widely employed. The first employs a vertical furnace inwhich a seed crystal of the material is contacted to the surface of amelt of the material and slowly withdrawn vertically therefrom. Themolten material attaches itself to the seed and solidifies thereon as itis withdrawn from the hot melting zone of the furnace. The crystallinecharacter and orientation of the seed determines the plane ofcrystallization of the solidifying material. This method is commonlyknown as the Kyropoulis-Czochralski technique. The second methodcomprises a horizontal furnace in which an elongated charge in contactwith a seed crystal is successively melted and refrozen by sweepingalong its length a melting zone of heat starting from the seed end ofthe charge. This method is usually referred to as zone-melting. In bothmethods a long single crystal is produced provided the seed employed issingle crystalline and properly oriented.

The type of conductivity that may be established in the semi-conductorcrystal is dependent upon the atomic structure of an impurity substancein relation to the atomic structure of the host crystal into which theimpurity is introduced. Thus a substance whose atoms are capable ofgiving up electrons in a particular host crystal is termed a donor, andsince there is a surplus of electrons available, the semi-conductor sodoped is deemed to be of n-type (negative) conductivity. n the otherhand, a substance whose atoms are capable of borrowing or acceptingelectrons in a particular host crystal is termed an acceptor, and sincethere is a shortage of electrons for current conduction, thesemi-conductor so doped is deemed to be of p-type (positive)conductivity.

In n-type semi-conductors current conduction is said to take place bythe available electrons; in p-type semi-conductors current conduction issaid to take place by holes. Thus in n-type semi-conductors theelectrons are termed majority charge carriers and holes are calledminority charge carriers. In p-type semi-conductors the holes are themajority charge carriers and electrons are the minority charge carriers.Transistor action is based on the ability to introduce minority chargecarriers into impurity doped semi-conductors by one means or another.The electric fields of injected holes or electrons will affect themajority charge carriers present in the semi-conductor so as toeventually result in a atent useful output current. However, electronsand holes in semi-conductor materials, being of opposite sign, tend torecombine; that is, the electrons tend to fill up the holes. In general,the longer the time before electrons and holes recombine, the more thenumber of uncombined charge carriers present for current conductionduring that time. Thus a measure of the usefulness of a particularsemi-conductor material is the lifetime of injected minority carriers bywhich is meant the time from injection to combination of minority chargecarriers with majority charge carriers. Generally, crystals with alifetime of about 50 microseconds and a resistivity of about 1 to 5ohm-cm. are preferred for manysemi-conductor devices.

Primary structural imperfections in the arrangement of the atoms ofsingle crystal semi-conductive materials have a marked effect upon thelifetime of the charge carriers. A characteristic type of crystallineimperfection is an edge-type dislocation which is apparently produced,for example, by the deformation process of slipthat is, as if some forcebears against the edge of a growing single crystal resulting in oneportion of the crystal being sheared relative to another portion. A morecomplete description of crystalline structural imperfections,particularly edge-type dislocations, is found in W. T. Reads bookentitled Dislocations in Crystals, published by McGraw-Hill Book Co.,New York, 1953, and with special reference to Chapter 1 thereof. Ingeneral, such imperfections result in a reduction in the carrierlifetime, and while not fully understood, it is generally believed thatthe recombination of holes and electrons is catalyzed by theseimperfections. Factors such as the growth rate of the crystal andtemperature gradients in the melt and in the growing crystal may allcontribute to such imperfections.

It is therefore an object of this invention to provide improved methodsand apparatus for growing single crystals of semi-conductive materialswith fewer structural imperfections such as edge-type dislocations andhaving increased charge carrier lifetime.

Another object of this invention is to provide improved methods andapparatus for growing single crystals of semi-conductive materialshaving increased charge carrier lifetime.

A still further object of this invention is to provide improved methodsand apparatus for growing single crystals of semi-conductive materialswith fewer structural imperfections such as edge-type dislocations.

Another object of the invention is to provide improved methods andapparatus for growing single crystals of germanium or silicon havingimproved charge carrier lifetime.

Another object of this invention is to provide improved methods andapparatus for growing single crystals of germanium and silicon withfewer structural imperfections such as edge-type dislocations and havingincreased charge carrier lifetime.

These objects and other advantages are achieved according to theinvention by growing a crystal from a melt and establishing a relativelysteep temperature gradient in the growing single crystal, which gradientresults in a higher charge carrier lifetime in the crystal, and bymaintaining substantially the entire interface between the melt and thesolidified growing crystal perpendicular to the axis of crystal growth,which interface results in a crystal having fewer structuralimperfections such as edge-type dislocations.

The steep temperature gradient is established along the length of thecrystal extending from the interface of the solid face of the crystal incontact with the melt. It has been discovered that the temperaturegradient maintained along the crystal in a longitudinal direction has amarked elfect upon the charge carrier lifetime of the crystal so grown.Furthermore, the steepness of the gradient is somewhat critical, theincrease in lifetime being significantly less when gradients of lessthan 100 C. per'cm. or more than 400 C. per cm. are used. Considerableimprovement in lifetime of charge carriers has been found with atemperature gradient for silicon within the range of 200 to 400 C. percm.; for germanium the preferred range is from 150 to 250 C. per cm. Toachieve a planar or flat solid-liquid interface normal to the avis ofcrystal growth, the temperature across the crystal diameter at itsinterface is maintained uniform or isothermal. Crystals grown accordingto the invention have dislocation densities ranging from to 100 cum- Theinvention will be described in greater detail With reference to thedrawings in which similar reference characters designate the same orsimilar elements throughout.

Figure 1 is a cross-sectional, elevational view of the growing end of asingle crystal showing the temperature gradient therealong according tothe invention;

Figure 2 is a partial cross-Sectional, elevational view of a verticalcrystal growing furnace having means to establish a particulartemperature gradient in a growing crystal;

Figure 3 is a partial cross-sectional, elevational view of a verticalcrystal growing furnace depicting an almost completely grown singlecrystal;

Figure 4 is a detailed cross-sectional, elevational view of a gas cooledseed crystal holder for a vertical crystal growing furnace such as shownin Figures 2 and 3;

Figure 5 is a detailed cross-sectional, elevational view of a liquidcooled seed crystal holder for a vertical crystal growing furnace suchas shown in Figures 2 and 3;

Figure 6 is a cross-sectional, elevational View of a horizontalzone-melting crystal growing furnace having means to establish aparticular temperature gradient in the growing single crystal at thebeginning of the crystal growing operation; and

Figures 7a, 7b, and 7c are cross-sectional, elevational views of thegrowing end of three crystals each of dif ferent diameters showing thelessening of curvature and. the increasing flatness of the interface ascrystal diameter is decreased.

Referring to Figure l, a seed crystal 2 of silicon is shown upon which agrowing single crystal 4 is attached as the seed is withdrawn from thesilicon melt 6. According to the invention, the single crystal 4 willhave a markedly higher carrier lifetime if grown under a temperaturegradient (as indicated) of between 200 and 400 C. per cm. This gradientis established along the first cm. of the growing crystal commencingwith the interface thereof between the solid and molten phases of thesilicon. Preferably the temperature gradient is about 300 C. per cm. inthe case of silicon. For germanium the temperature gradient lies withinthe range of 150 to 250 C. per cm. and preferably is about 200 C. percm.

The function of these critical temperature gradients in obtaining higherlifetimes is not fully understood. It is believed, however, that thepresence of the relatively sharp gradient prevents the condensation orformation of clusters of vacancies. (A vacancy occurs when a crystallattice site is not occupied by an atom of the crystal material.) It isthought that the aggregation or clustering of vacant lattice sites inthe atomic arrangement of single crystals of germanium and siliconresults in the ultimate collapse of disc-like holes in the crystalstructure. The collapse of these holes, in turn, is thought to producedislocations and other carrier lifetime-damaging imperfections incrystals. By establishing the above-indicated temperature gradients itis believed that the time during which vacancies are mobile and able tocluster is so short that these vacancies are frozen in situ and thusisolated. However, the invention is not predicated upon the correctnessof this theory.

It is also thought that a non-uniform temperature distribution acrossthe crystal at the interface between the liquid and solid phases in adirection normal to the axis of crystal growth results in the generationof localized' shear strains. These shearfstrains are believed to bringabout deformation by slip during crystal growth which is considered tobe the main source of edge-type dislocations. Hence it is necessary tomaintain the liquid-solid interface as flat or planar as possible withrespect to the direction of crystal growth. Therefore the thermalgradient established in the solidified crystal at the interface endthereof should be uniform across the crystal so that the temperature atthe center portion of the crystal interface region is the same as thetemperature at the peripheral portions of the growing crystal.

The advantages and significance of the invention can be It will beunderstood that the above measurements were taken on the crystal asgrown and were made on the first portion to solidify. It should also benoted that in all cases a slight decrease in resistivity of the crystalwas observed. This decrease in resistivity is apparently due to the lossin the final crystal by segregation of certain impurities present in theoriginal crystal. Thus with a gradient of about 300 C. per cm. a singlecrystal is obtained whose lifetime is between 40-70 microseconds andwhose resistivity is about 5 ohm-cm.

The effect on the dislocation density in a crystal grown according tothe invention wherein the interface between the liquid-solid phases ismaintained flat or planar is shown by the following data. The averageedge-type dislocation density in vertically pulled crystals Where noeffort is made to control the shape of the interface is between 10 and10 cmr' Crystals have been grown according to the invention with a flatinterface having dislocation densities ranging from 0 to cmr Because ofthe difiiculty in establishing a radially uniform temperature in a thickgrowing crystal, it is difiicult to establish the requisite flat orplanar interface. However, by growing crystals of smaller diameters itis easier to extract heat from the interface region uniformly across thecrystal and thus establish the flat interface. A silicon crystal havinga diameter of 1" had an edge dislocation density of 10 cmr a siliconcrystal having a diameter of 0.5" had an edge dislocation density of 10cm.- and having a diameter of 0.25" had an edge dislocation density ofsubstantially 0 ems- It should be appreciated that the diameter size ofthe crystal grown according to the invention is not the contributingfactor in obtaining dislocation-free crystals. As a practical matter itwas found easier to establish the flat interface in crystals of smalldiameter for thermodynamic reasons. According to the invention a crystalof any diameter can be grown dislocation-free if the interface betweenthe liquid-solid phases is maintained fiat. Thus it was observed that ascrystal diameter decreased the interface between the solid grown portion4 of the crystal and the molten portion 6 became flatter and flatter asshown in Figure 7. This is due, as noted previously, to the fact thatthe temperature distribution across the crystal is much more uniform inthe small diameter crystal as compared with the temperature distributionin larger crystals.

The necessary temperature gradients may be obtained by any number ofconvenient methods. It is preferred, according to the invention, toextract the heat from the growing crystal to establish the desiredgradient by simultaneously cooling both the portion of the crystalimmediately adjacent the interface and the far or seed end of thecrystal. At the commencement of crystal growth it was found that merelycooling the seed end of the crystal was sufficient to achieve thenecessary gradient. However, as the length of the crystal increased theextraction of heat from the interface region becomes more and moredependent upon thermal conduction through the crystal. Hence it becomesnecessary to eventually extract the heat more directly from the regionof the interface.

A vertical Kyropoulis-Czochralski type of crystalgrowing furnace isshown in Figure 2. A crucible 8, preferably of quartz, is containedwithin a carbon crucible 10. An electrical induction heating coil 12surrounds the crucible assembly and is provided with an alternatingelectric current from any convenient power source (not shown). A mass ofsilicon 6 is placed within the quartz crucible and melted. The carboncrucible 10 is partially contained within the lower end of a quartz orheat-resistant glass tube 18 which extends vertically therefrom. The topend of the tube 18 is closed by a lid 20 or an air-tight metal fixturewhich may also be of quartz. Into the top end of the tube 18 and throughan aperture in the lid 20 an elongated shank member 22 extendsdownwardly. The shank 22 has a seed holder 24 on its lower end to whichmay be attached a seed crystal 2 or" silicon. The upper end of the shankmember is attached to a wire or cord 28 passing over pulleys 30 and 32to a drum 34 driven through a chain of reduction gears within a gear box36 by a reversible motor 38. A gear train 2123 couples the shank to amotor 25 which rotates the shank during crystal growth.

The operation of the apparatus is as follows: The crucible 8 is chargedwith material from which a crystal 4 is to be grown. The charge maycomprise silicon in the form of fine needles, powder, or small pieces.Upon energization of the induction heating coil 12 the charge melts.When the silicon is completely molten, the seed crystal holder 24 withthe seed crystal 2 attached is carefully lowered until it just touchesthe surface of the molten silicon. The motor 33 is then started so thatit begins to slowly wind the cord 28 on the drum 34. Thus, the shankmember 22 is slowly raised at a speed of about 0.5 mm. per minute. Theshank 22 and hence the growing crystal 4 are rotated at a speed of about50 r.p.m. The speed of upward movement of the shank member and seedcrystal holder is adjusted so that a silicon crystal 4 grows attached tothe seed crystal 2 and continues to grow as long as the silicon isavailable in the crucible. The temperature of the melt surface isadjusted almost to the freezing point of silicon (about 1420" C.).

In order to establish the desired temperature gradient of about 300 C.per cm. at the start of the crystal pulling operation, the seed end ofthe crystal is cooled by a jet of inert gas directed against the seedcrystal 2. This may be accomplished by means of a hollow shank 22supporting a hollow seed holder 24 such as shown in detail in Figure 4.The seed holder may be of metal and is tapered inwardly at its bottomend so as to suspend a seed crystal with a widened end as shown.Alternatively, the seed crystal is held in place by means of set screws(not shown). An inert gas such as argon or helium is introduced into theseed holder 24 and against the seed crystal through a pipe 27 whichextends downward through the hollow shank 22. A special coupling device29 is shown for delivering the gas from a stationary source (not shown)to the pipe 27 located within the rotating shank 22. The shank isannularly grooved. A hole is drilled from the groove into the cavitywithin the shank. The pipe 27 is inserted in this hole. A gas-tightstationary bushing 31 encloses the grooved portion of the shank. Thebushing has a hole drilled within it which is aligned with the groove ofthe shank. The gas source is connected bp a pipe inserted in this hole.Thus as the shank turns, there'is always a connection from the gassource to the pipe through the groove-connected channels in the bushingand the shank.

As mentioned previously, cooling the seed end of the crystal issufiicient to establish the required gradient while the crystal is ofrelatively short length. However, as the crystal grows and the distancebetween the cooling gas jet and the crystal interface increases itbecomes increasingly diflicult to maintain the gradient. Therefore, analternative cooling arrangement is provided which is preferably employedin combination with the cooling of the seed end of the crystal. Jets ofhelium or argon or another inert gas are played upon the regionimmediately adjacent the crystal interference by means of a jet 33 onthe rotating end. The gas is supplied to the jet 33 by means of the pipe52 which enters the crucible chamber through an aperture in the cover20.

As shown in Figures 2 and 3, as the crystal grows, the melt becomesdepleted and falls to an increasingly lower level in the crucible. Hencethe jet 33 must be slowly lowered into the crucible chamber so as tomaintain the jet of gas on the portion around the region of the crystalinterface.

An important alternative method for cooling the seed end of the crystalis shown in Figure 5. Here the hollow shank 22 has its lower end closedoff and in intimate contact with the upper end of the seed crystal 2.Water is introduced and circulated through the hollow shank by means ofthe pipe 54 which runs down through the shank. The outlet of the pipe 54into the hollow shank is located down near the region of the shank whichcontacts the seed crystal. Thus relatively cool water enters the pipe 54and leaves through the shank 22. Heat is extracted by the water from theseed crystal 2 by conduction through the shank wall in contacttherewith.

The coupling device for admitting and removing the water is based on thesame principle as the gas coupling device (29) described in connectionwith Figure 4. The hollow shank 22 contains an inner pipe 54. The hollowseed holder 24 may be threaded onto the lower end of the shank 22. Thepipe 54 extends downwardly from within the shank and into the seedholder so as to deliver the water adjacent the end of the seed crystal2. The upper end of the hollow shank has a passage drilled through itswall which connects the internal cavity of the shank with an annulargroove on its outside surface. The inlet pipe 54 is threaded into thesmaller diameter portion of the upper part of the shank and a passage isdrilled through the shank to connect this pipe to a second annulargroove in the outer surface of the shank. The stationary bushing 35 hastwo passages drilled through it so as to connect water inlet and outletpipes 37-39, with the respective inlet and outlet grooves of the shank22. In this manner Water or any other coolant may be introduced into andcirculated within the rotating seed holder 24.

The practice of the invention both as to the requisite temperaturegradient and interface flatness may also be carried out when growingcrystals by the horizontal zone melting technique. Referring to Figure6, an elongated boat-like crucible 8 of graphite, for example, ischarged with a rod shaped piece of germanium 14, for example. At one endof the germanium rod a seed crystal 2 of germanium is placed.

The crucible 8 is placed within a quartz tubular enclosure 58 and isgradually propelled at about 2.5 inches per hour or less, for example,through a ring shaped induction heating element 12 starting at the endof the crucible where the seed crystal 2 is placed. The crucible issupported within the quartz tube 58 by means of a ring shaped member 61.When the temperature reaches the melting point of germanium (about 940C.), the initial end of the germanium rod melts as well as a portion ofthe seed crystal 2. As the crucible with its contents is propelledthrough the heating element 10 successive segments of the rod 14 aremelted and refrozen. The refrozen portions grow as a single crystalextension of the seed crystal.

In order to establish a temperature gradient of about 200 C. per cm.along the length of the growing crystal jets of an inert gas such asargon are played upon the end of the seed crystal 2 and against theregion of the growing crystal immediately adjacent the interface. Thismay be accomplished by means of the nozzles 60 and 62 which areconnected to a source of inert gas (not shown). The jet of gas which isbeing directed against the region of the crystal interface should becarefully controlled so as to not strike the molten portion 64. If thegas strikes the molten portion, the surface thereof will probably freezein advance of the solid-liquid interface and will increase thedifiiculty of propagating single crystalline growth due to the undesirednucleating center formed by the solidified surface.

There thus has been shown and described an improved method and apparatusfor growing single crystals of semi-conductive materials such asgermanium and silicon, having greatly increased carrier lifetime as wellas being freer of edge-type dislocations. The invention should not belimited to the specific apparatus shown and described for the purpose ofillustrating the invention. Many modifications of the embodimentsdescribed can be made without departing from the spirit of the inventionwhich modifications include the establishment of a particulartemperature gradient range and a flat or planar interface between theliquid-solid phases of a growing crystal in order to obtain greatlyimproved single crystals of semiconductive materials. With respect tomaintaining a flat interface when growing crystals, it should beunderstood that the invention is concerned with the physical orcrystallographic structure of materials rather than upon the chemistrythereof. Hence this aspect of the invention may be practiced toadvantage in growing crystals of any material and is not limited to thespecific exemplary materials described in this specification, namelygermanium and silicon.

What is claimed is:

l. The method of growing a single crystal of a semiconductive materialcomprising the steps of: melting at least a portion of said material,contacting a seed crystal of said material to said molten portion,relatively moving said seed crystal with respect to said molten portionwhereby successive portions of said molten portion are frozenout on saidseed crystal, establishing and maintaining in said frozen-out portion atemperature gradient of between 150 and 400 C. per cm. adjacent theinterface between said frozen solid portions and said melt, andmaintaining the temperature across said frozen solid portions radiallyuniform whereby said interface is planar in a direction normal to thedirection of crystal growth.

2. The method according to claim 1 wherein said semiconductive materialis silicon and said temperature gradient is between 200 and 400 C. percm.

3. The method according to claim 1 wherein said semiconductive materialis germanium and said temperature gradient is between 150 and 250 C. percm.

4. The method of growing a single crystal of a semiconductive materialcomprising the steps of: melting at least a portion of said material,contacting a seed crystal of said material to said molten portion,relatively moving said seed crystal with respect to said molten portionwhereby successive portions of said molten portion are frozen-out onsaid seed crystal, and establishing and maintaining in said frozen-outportion a temperature gradient of between and 400 C. per cm. adjacentthe interface between said frozen solid portions and said melt.

5. The method according to claim 4 wherein said semiconductive materialis silicon and said temperature gradient is between 150 and 250 C. percm.

6. The method according to claim 4 wherein said semiconductive materialis germanium and said temperature gradient is between 150 and 250 C. percm.

7. In the vertical pulling method for growing single crystals of asemi-conductive material selected from the class consisting of germaniumand silicon, the method of growing high lifetime substantiallydislocation-free crystals of said material comprising the steps of:forming a melt of said material, contacting a seed crystal of saidmaterial to the surface of said melt, slowly withdrawing said seedcrystal at a predetermined rate so as to grow a solidified crystalportion of said molten material attached to said crystal, establishing aconstant temperature gradient along the length of said solid crystalportion and said seed crystal of between 150 and 400 C. per cm. from theinterface between said solid crystal portion and said melt, maintainingthe temperature across said solidified crystal portion radially uniformwhereby said interface is planar in a direction normal to the directionof crystal growth, and continuing to withdraw said growing crystalportion to continue the growth thereof while maintaining said gradientand said planar interface.

8. In the vertical pulling method for growing single crystals of asemi-conductive material selected from the class consisting of germaniumand silicon, the method of growing high lifetime crystals of saidmaterial comprising the steps of: forming a melt of said material, contacting a seed crystal of said material to the surface of said melt,slowly withdrawing said seed crystal at a predetermined rate so as togrow a solidified crystal portion of said molten material attached tosaid crystal, establishing a constant temperature gradient along thelength of said solid crystal portion and said seed crystal of between150 and 400 C. per cm. from the interface be tween said solid crystalportion and said melt, and continuing to withdraw said growing crystalportion to continue the growth thereof while maintaining said gradient.

9. The method of claim 1 wherein said step of relatively moving saidseed crystal with respect to said molten portion is accomplished byhorizontally zone melting said semiconductive material.

10. The method of claim 9 wherein said semiconductive material issilicon and said temperature gradient is between 200 and 400 C. percentimeter.

11. The method of claim 9 wherein said semiconductive material isgermanium and said temperature gradient is between 150 and 250 C. percentimeter.

Physical Review (2nd series), vol. 33, 1929, pages 81 to 89. Publishedby the American Physical Society, Minneapolis, Minn.

Schmid et al.: Plasticity of Crystals, page 31, January 1950.

1. THE METHOD OF GROWING A SINGLE CRYSTAL OF A SEMICONDUCTIVE MATERIALCOMPRISING THE STEPS OF: MELTING AT LEAST A PORTION OF SAID MATERIAL,CONTACTING A SEED CRYSTAL OF SAID MATERIAL TO SAID MOLTEN PORTION,RELATIVELY MOVING SAID SEED CRYSTAL WITH RESPECT TO SAID MOLTEN PORTIONWHEREBY SUCCESSIVE PORTIONS OF SAID MOLTEN PORTION ARE FROZEN-OUT ONSAID SEED CRYSTAL, ESTABLISHING AND MAINTAINING SAID FROZEN-OUT PORTIONA TEMPERATURE GRADIENT OF BETWEEN 150 AND 400*C. PER CM. ADJACENT THEINTERFACE BETWEEN SAID FROZEN SOLID PORTIONS AND SAID MELT, ANDMAINTAINING THE TEMPERATURE ACROSS SAID FROZEN SOLID PORTIONS RADIALLYUNIFORM WHEREBY SAID INTERFACE IS PLANAR IN A DIRECTION NORMAL TO THEDIRECTION OF CRYSTAL GROWTH.